In an attempt to target cytotoxic drugs to tumour cells daunomycin-low density lipoprotein (LDL) complexes were synthesised. Human squamous lung tumour cells in vitro have large numbers of high affinity cell surface LDL receptors (Vmax = 19 ng LDL/micrograms cell protein per 24 hr; Km = 23 micrograms/ml). Cellular uptake of daunomycin and LDL-daunomycin was rapid and approached equilibrium by approximately 3 hr. Intracellular daunomycin concentrations were similar at each time point regardless of whether free drug or the complex was used. The degree of intracellular drug metabolism differed markedly with significantly higher production of daunomycinol following exposure to free daunomycin for 90 min. Daunomycin and LDL-daunomycin were equally cytotoxic in vitro (respective clonogenic ID90s of 1 microgram/ml and 0.7 microgram/ml). Fluorescence microscopy indicated that both free daunomycin and LDL-daunomycin have a punctate, granular distribution within the cytoplasm.